Vehicle suspension systems may include ride height control systems for controlling the height of a vehicle relative to a surface on which the vehicle rides. Existing ride height control systems generally operate based on the position of the sprung mass (e.g., the vehicle chassis) relative to the unsprung mass (e.g., the vehicle wheels or axle). A ride height control system may monitor and adjust the position at each wheel location to provide a target ride height. The target ride height may be controlled by the operator manually, for example, by selecting pre-set target ride heights. The target ride height may also be controlled dynamically based on vehicle operating conditions (e.g., speed).
Ride height control may be provided in suspension systems, such as air spring suspension systems, hydropneumatic suspension systems and steel spring systems, in which the ride height is dependent upon the volume of air and/or hydraulic fluid in the system. In hydropneumatic suspension systems, for example, the ride height may be increased by pumping hydraulic fluid (e.g., mineral oil) into the system at each of the wheel locations. The ride height is often controlled by adding or removing air/fluid at each of the wheel locations independently to achieve a target ride height position at that wheel location. In the existing ride height control systems, which control ride height based only on position, the target ride height may be achieved at the wheel locations but in any vehicle with more than three wheels there exist many possible load conditions with the wheels at the correct height. As a result, different pressures in the suspension system at different wheel locations can occur and thus different loads are supported by the suspension system at different wheel locations, unnoticed by the operator. These problems may be compounded in vehicles with a larger number of wheels (e.g., eight wheel vehicles) such as those used by the military.
One problem that occurs in vehicle suspension system with existing ride height control systems is a cross-jacking condition. Under a cross-jacking condition, a load imbalance exists at the wheel locations even though the vehicle may be at the target ride height. When ride height is controlled only based on position, the target ride height may be achieved but with significantly higher loads at some of the wheel locations compared to other wheel locations. The suspension system at the wheel locations at two opposite corners, for example, may be pressurized significantly higher (e.g., 90 psi) than the wheel locations at the other two opposite corners (e.g., 30 psi) while still maintaining the same ride height. In terms of the vehicle load, two of the opposite corner wheel locations may each support 1500 lb. of a 4000 lb. vehicle and the other two opposite corner wheel locations may each support 500 lb. These load imbalances may adversely affect the handling of the vehicle, for example, as a result of different tractions on the tires and/or different suspension response at the different wheel locations.
Another problem may occur in vehicle suspension systems with existing ride height control systems as a result of a wheel displacement condition. Under a wheel displacement condition, one (or some subset) of the wheels of a vehicle may be stopped on a surface irregularity that causes the wheel(s) to be displaced in an upward or downward direction. If the vehicle is stopped with a wheel on a rock or curb, for example, the wheel will be displaced in an upward direction. If the vehicle is stopped with the wheel in a ditch, the wheel will be displaced in a downward direction.
The upward displacement, in particular, will cause compression of the suspension system at that location and the wheel and chassis will have a closer position with a higher pressure and load at that wheel location. Existing ride height control systems that control ride height based only on position will interpret this closer position as requiring an adjustment to raise the ride height at that location in an attempt to maintain the target ride height. The ride height is raised, for example, by adding air or fluid to the suspension system at the upwardly displaced wheel location. The position at the other locations may then be independently adjusted to maintain the target ride height, which may result in different pressures and loads at the different wheel locations. When the vehicle is moved back to a relatively flat surface after these adjustments, the loads may be unbalanced and the ride height may initially need to be readjusted. These load imbalances and ride height adjustments caused by wheel displacement conditions may also adversely affect the vehicle performance and handling.